Moving laser beam readers or laser scanners have long been used as data capture devices to electro-optically read targets, such as one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, printed on labels associated with products in many venues, such as supermarkets, warehouse clubs, department stores, and other kinds of retailers, as well as many other venues, such as libraries and factories. The moving laser beam reader generally includes a housing, a laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances relative to the housing, a scan component for repetitively scanning the beam spot across a target in a scan pattern, for example, a scan line or a series of scan lines, across the target multiple times per second, and a photodetector for detecting return laser light reflected and/or scattered from the target and for converting the detected return laser light into an output analog electrical information signal bearing information related to the target. This analog electrical information signal varies in amplitude as a function of time due to the time-varying return laser light along each scan line, and varies in frequency as a function of the density of the symbol, as well as the distance at which the symbol is being read. The moving laser beam reader also includes signal processing receiver circuitry including a digitizer for digitizing the variable analog information signal, and a microprocessor for decoding the digitized signal based upon a specific symbology used for the target. The decoded signal identifies the product and is transmitted to a host, e.g., a cash register in a retail venue, for further processing, e.g., product price look-up or product inventorying.
In one advantageous embodiment, during operation of the moving laser beam reader in a venue having one or more external light sources that emit ambient light, an operator holds the housing in his or her hand, and aims the housing at the target, and then initiates the data capture and the reading of the target by manual actuation of a trigger on the housing. The ambient light is also concomitantly detected by the photodetector, which generates an analog electrical ambient light signal. In the event that the external source is sunlight, then the ambient light is substantially constant in magnitude, and therefore, the analog electrical ambient light signal has a constant illumination DC component. In the event that the external source is an incandescent bulb or a fluorescent lamp energized at 50 Hz or 60 Hz, then the analog electrical ambient light signal has a constant illumination DC component and a relatively small time-varying AC frequency component at 50 Hz or 60 Hz. In the event that the fluorescent lamp is operated at higher frequencies for greater luminous efficiency, or in the event that the external source includes light emitting diodes (LEDs) operated at higher frequencies, then the analog electrical ambient light signal has a constant illumination DC component and a relatively larger time-varying AC frequency component at kilohertz frequencies, typically anywhere from 30 kHz to 300 kHz.
In some circumstances, the presence of the ambient light signal interferes with, and weakens, the information signal. To prevent such interference, the constant illumination DC component of the ambient light signal can generally be filtered out from the information signal. Also, filters can be used to suppress the ambient light signal when its time-varying frequency component is very far in frequency away from the frequency of the information signal. However, if the time-varying frequency component of the ambient light signal is too close in frequency to the frequency of the information signal, then the ambient light signal can interfere and impede the decoding of the information signal, thus degrading the performance of the reader. By way of non-limiting example, an information signal of about 50 kHz can be generated during reading of a low density symbol located relatively close to the reader, e.g., about 10 inches away. If the ambient light source includes LEDs operated at about 50 kHz, then the respective frequencies of the ambient light signal and the information signal are too close and will cause an interference, and perhaps cause the symbol not to be successfully decoded and read.
To prevent such interference, it is also known to reduce the ambient light signal by placing a detector lens in front of the photodetector. The detector lens can be implemented either as a transmissive or a reflective component. The detector lens limits the field of view (FOV) of the photodetector. The FOV is minimized and limited to desirably include substantially only the laser beam spot. Still, even within this limited FOV, the ambient light signal can still be too high for successful decoding.
To detect and measure the magnitude of the ambient light signal, it is known to analyze an output analog electrical signal of the photodetector when the laser is deenergized, i.e., when there is no information signal. This typically occurs at the start of a reading session, e.g., during a start of scan (SOS) frame, which, for example, advantageously lasts for about 16 milliseconds. When the laser is deenergized, however, the target cannot be read, and the reader must wait for the laser to be energized. If the ambient light signal is, for some reason, not detected during this initial deenergization of the laser, then the reader must wait for the next time that the laser is deenergized to attempt the detection of the ambient light signal again. All this waiting negatively impacts the reader's aggressiveness and renders its performance sluggish.
Accordingly, there is a need to suppress such interference caused by such ambient light and to enhance reader performance without having to deenergize the laser.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.